Diode lasers output radiation at one frequency or frequency interval, further the diode laser may emit radiation at a further frequency or frequency interval, but that frequency, frequencies or frequency interval may not be the desired frequency interval. For example, when a diode laser is used for pumping another laser, the output of the diode laser apparatus needs to match the acceptance frequency band of the laser to be pumped. Therefore there is a need for providing a system that allows the emitted radiation to be transformed to the desired frequency or frequency interval. The term frequency and wavelength may be interchanged throughout the description using the physical relation between frequency and wavelength.
The article “Frequency-doubled DBR-tapered diode laser for direct pumping of Ti-sapphire lasers generating sub-20 fs pulses” by André Müller et al., Optics Express, Vol. 19, 12156, 2011, has demonstrated that such a laser system can provide power levels that enable competitive direct optical pumping. The use of diode lasers as direct pump source for Ti:sapphire lasers allows the development of low-cost, ultrafast lasers with high efficiencies and small footprints. It will further be appreciated that frequency-doubled diode laser systems may be applied in a variety of alternative applications, e.g. as light source in a measuring system, display systems, medical and other diagnostic systems, etc.
Nevertheless, it is generally desirable to increase the output power of such a laser system.
Optical frequency mixing and, in particular, second harmonic generation (SHG), has been suggested to allow a single laser source to be used for multiple operations, e.g. as described in U.S. Pat. No. 6,441,948. Optical frequency mixing often attempts to generate higher power harmonics of solid state lasers such as Nd:YAG lasers in the deep UV range. This prior art document further discusses the importance of properly matching the refractive indeces for enhancing the non-linear process, the so-called phase matching condition. In particular, this prior art method proposes the use of a series of non-linear crystals wherein each crystal is independently adjusted to compensate for thermally induced phase mismatch. In this way, the phase mismatch is always less than π in each crystal. Even though, this prior art method provides a compensation for thermally induced phase mismatch in the individual crystal, it remains a problem to provide an overall improvement of the conversion efficiency of the overall system.
D. Fluck and P. Günter, “Efficient second-harmonic generation by lens wave-guiding in KNbO3 crystals,” Optics Communications, vol. 147, pp. 305-308, February 1998 discloses another example of a cascade of crystals performing second harmonic generation (SHG). The system of Fluck exploits the fact that a fixed phase relation between the fundamental beam and the SHG beam may be maintained.
A different optical frequency mixing technique involves the optical frequency mixing of laser beams from different sources where the incoming laser beams do not necessarily have the same frequency or are harmonics of each other. In particular, sum frequency generation (SFG) generates a combined laser beam from a first and a second laser beam such that the combined beam has a frequency given by the sum of the frequency of the first and second beams. However, as sum frequency generation requires an arrangement with two input beams, this technique has previously been regarded as more complex. In particular, in the SFG case and in contrast to the SHG, the total output power of the combined beam is a function of three phases, namely the respective phases of the incoming beams and of the combined beams. In contrast to the second harmonic generation, a fixed phase relation can thus not easily be maintained between all three beams.
In view of the prior art, it thus remains desirable to increase the output power of such a laser system.